1. Field of the Invention
This invention relates to the synthesis of boronic ester and acid compounds. More particularly, the invention relates to large-scale synthetic processes for the preparation of boronic ester and acid compounds by Lewis acid promoted rearrangement of boron “ate” complexes.
2. Background of the Invention
Boronic acid and ester compounds display a variety of pharmaceutically useful biological activities. Shenvi et al., U.S. Pat. No. 4,499,082 (1985), discloses that peptide boronic acids are inhibitors of certain proteolytic enzymes. Kettner and Shenvi, U.S. Pat. No. 5,187,157 (1993), U.S. Pat. No. 5,242,904 (1993), and U.S. Pat. No. 5,250,720 (1993), describe a class of peptide boronic acids that inhibit trypsin-like proteases. Kleeman et al., U.S. Pat. No. 5,169,841 (1992), discloses N-terminally modified peptide boronic acids that inhibit the action of renin. Kinder et al., U.S. Pat. No. 5,106,948 (1992), discloses that certain tripeptide boronic acid compounds inhibit the growth of cancer cells.
More recently, boronic acid and ester compounds have displayed particular promise as inhibitors of the proteasome, a multicatalytic protease responsible for the majority of intracellular protein turnover. Ciechanover, Cell, 79: 13-21 (1994), discloses that the proteasome is the proteolytic component of the ubiquitin-proteasome pathway, in which proteins are targeted for degradation by conjugation to multiple molecules of ubiquitin. Ciechanover also discloses that the ubiquitin-proteasome pathway plays a key role in a variety of important physiological processes.
Adams et al., U.S. Pat. No. 5,780,454 (1998), U.S. Pat. No. 6,066,730 (2000), U.S. Pat. No. 6,083,903 (2000), U.S. Pat. No. 6,297,217 (2001), U.S. Pat. No. 6,548,668, and U.S. Pat. No. 6,617,317 (2003), hereby incorporated by reference in their entirety, describe peptide boronic ester and acid compounds useful as proteasome inhibitors. The references also describe the use of boronic ester and acid compounds to reduce the rate of muscle protein degradation, to reduce the activity of NF-κB in a cell, to reduce the rate of degradation of p53 protein in a cell, to inhibit cyclin degradation in a cell, to inhibit the growth of a cancer cell, to inhibit antigen presentation in a cell, to inhibit NF-κB dependent cell adhesion, and to inhibit HIV replication.
Albanell and Adams, Drugs of the Future 27: 1079-1092 (2002), discloses that one such peptide boronic acid proteasome inhibitor, bortezomib (N-2-pyrazinecarbonyl-L-phenylalanine-L-leucineboronic acid), shows significant antitumor activity in human tumor xenograft models and is undergoing clinical evaluation. Richardson et al., New Engl. J. Med., 348:2609 (2003), reports the results of a Phase 2 study of bortezomib, showing its effectiveness in treating relapsed and refractory multiple myeloma.
Studies of boronic acid protease inhibitors have been greatly advanced by the development of chemistry for the preparation of functionalized boronic add compounds, particularly alpha-halo- and alpha-aminoboronic acids. Matteson and Majumdar, J. Am. Chem. Soc., 102:7558 (1950), discloses a method for preparing alpha-chloroboronic esters by homologation of boronic esters, and Matteson and Ray, J. Am. Chem. Soc., 102:7590 (1980), reports that chiral control of the homologation reaction can be achieved by the use of pinanediol boronic esters. The preparation of alpha-aminoboronic acid and ester compounds from the corresponding alpha-chloroboronic esters has also been reported (Matteson et al., J. Am. Chem. Soc., 103:5241 (1981); Shenvi, U.S. Pat. No. 4,537,773 (1985)).
Matteson and Sadhu, U.S. Pat. No. 4,525,309 (1985), describes an improved procedure for the homologation of boronic esters by rearrangement of the intermediate boron “ate” complex in the presence of a Lewis acid catalyst. The Lewis acid is reported to promote the rearrangement reaction and to minimize epimerization at the alpha-carbon atom. Rigorous exclusion of water and careful control of Lewis acid stoichiometry are required for optimum results, however. These features render the reaction difficult to perform successfully on a production scale, and limit the availability of pharmaceutically important boronic ester and acid compounds, such as bortezomib. Thus, there remains a need in the art for improved methods for the large-scale production of boronic ester and acid compounds.